This invention relates to inertial guidance systems and more particularly, to systems for maintaining an inertial platform in a substantially fixed spatial orientation.
Inertial guidance systems for vehicles with unlimited angular freedom (e.g. airborne or spaceborne vehicles) require an inertial reference upon which the orientation control mechanization is based. Such a reference is generally provided by a platform which is maintained in a relatively stable spatial orientation, irrespective of the vehicle motion. A system for maintaining the platform in that stable orientation over a predetermined range of vehicle motions may take the form of a nested configuration of a platform with two or more successively larger and concentrically disposed gimbals and associated driving means and control circuitry. However, there are certain relative platform and gimbal orientations which must be avoided in order to maintain the platform stabilization system in an operative state.
When the platform and gimbals are in the orientation-to-be-avoided, the system is said to be in "gimbal lock". The gimbal lock state is defined as being the state in which the axes of rotation of all the gimbals and the platform lie in a single plane. In this situation, it is impossible for the platform stabilization system to maintain the platform in its substantially fixed orientation when the vehicle has an angular velocity with a component perpendicular to the plane of the rotation axes.
A three axis gyro stabilized inertial reference system (having a nested configuration of a platform and two gimbals) provides for the three degrees of freedom required by the generalized motion of a vehicle. If the vehicle motion is restricted to avoid gimbal lock, then such a three axis system is adequate to provide an inertial reference for a navigation or a guidance control system. However, in a nested three axis system, gimbal lock occurs whenever the outermost axis of rotation (fixed to the vehicle) becomes parallel to the innermost axis of rotation (fixed to the platform). Thus, three axis systems are subject to inherent constraints on the vehicle orientation in order to avoid the gimbal lock state.
To eliminate such restrictions of vehicle motion while at the same time avoiding the possibility of gimbal lock, platform stabilization systems having a platform and three or more gimbals (with associated rotational axes) may be utilized with platform and gimbal motion only three of the axes being controlled at any one time to maintain the platform stable, while the remaining axis (or axes) are redundant. That is, the relative orientation of three of the rotatable elements (the platform and the gimbals) are continuously adjusted to maintain the platform in a substantially fixed spatial orientation as though the system included only the platform and two gimbals (as in a three axis system), while the relative orientation of the remaining gimbal (or gimbals) is adjusted to prevent the alignment of all gimbal axes in the same plane. Thus, in a four or more axis system, gimbal lock may be avoided while permitting the vehicle to have unrestricted angular orientation through the appropriate assignment and control of selected ones of the gimbals about their associated rotational axis to be utilized in a stabilization control loop.
The classical techniques for embodying a redundant gimbal system utilize the platform and innermost gimbal and select one of the two outermost gimbals to be in the stabilization loop while the non-selected one of the two outermost gimbals is constantly driven to keep that redundant gimbal near its null position.
Four axis systems of this type are well known in the art. See, for example, Macomber and Fernandex, Inertial Guidance Engineering, Prentice-Hall, Englewood Cliffs, N.J., 1962, Chapter 3. However, as noted by Macomber and Fernandez, such systems are subject to the disadvantage that, during certain vehicles maneuvers, the angular velocity of certain of the gimbals is required to be infinite for perfect stabilization of the platform. Furthermore, this infinite angular velocity requirement is not a singular occurrence, but occurs at a plurality of points. Of course, in practical embodiments, the platform is less than perfectly stabilized, but relatively large and rapid changes in angular position of certain of the gimbals, or "gimbal flip", are still required to maintain acceptable levels of platform stabilization.
As a result of the inability of realizable systems to achieve the infinite acceleration required for perfect stabilization, all systems represent trade-offs in performance. For example, redundant gimbal flips are required in prior art systems to be as great as 180.degree. during certain vehicle maneuvers accommodating a predetermined range of vehicle angular rates while at the same time maintaining the platform at an "acceptable"level of stability. In order to achieve the relatively large flips in the required short intervals necessary to meet the stability requirements, substantially high torque and consequently, high power mechanizations may be required to drive the gimbals. Of course, in airborne and spaceborne vehicles, both element size of all components and power requirements are desired to be minimized.
On the other hand, the same level of platform stability may be attained in alternative prior systems with smaller, lighter torque mechanizations provided the permitted range of vehicle angular rates is correspondingly reduced. Of course, the permitted range of angular rates may be traded-off with component power requirements to any desirable degree. Thus, the prior art systems encounter relatively severe restrictions in component size and power and permitted range of vehicle angular rates for a predetermined level of platform stabilization.
Accordingly, it is an object of the present invention to provide a four axis gyro stabilized inertial reference system for maintaining an inertial platform in a substantially fixed spatial orientation while minimizing the magnitude of required gimbal flips.
A further object is to provide a system utilizing a four axis gyro stabilized platform and control system for maintaining a platform in a substantially fixed spatial orientation while limiting the magnitude of redundant gimbal flips to be approximately 45.degree. or less during any permitted vehicle maneuver.
A still further object is to provide a four axis gyro stabilized inertial reference system including a control system for selecting the stabilization axes in response to the detected relative inner and middle gimbal angles in the system.
Still another object is to provide a system for maintaining an inertial platform in a substantially fixed spatial orientation requiring relatively small torque and power mechanizations for gimbal motion for a predetermined range of permitted vehicle angular rates.
Another object is to provide a system for maintaining an inertial platform in a substantially fixed spatial orientation while permitting relatively large ranges of vehicle angular rates for predetermined torque and power mechanizations.